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Abstract

Multiphase and non-Darcy flow effects in hydraulically fractured gas wells reduce
effective fracture conductivity. Typical proppant pack laboratory experiments are
oriented in such a way such that phase segregation is not possible, which results in
mixed flow. Tidwell and Parker (1996), however, showed that in proppant packs, gravity
segregation occurs for simultaneous gas and liquid injection at laboratory scale (1500
cm2). Although the impact of gravity on flow in natural fractures has been described,
previous work has not fully described the effect of gravity on multiphase non-Darcy
flow in hydraulic fractures. In this work, reservoir simulation modeling was used to
determine the extent and impact of gravity segregation in a hydraulic fracture at field
scale. I found that by ignoring segregation, effective fracture conductivity can be
underestimated by up to a factor of two.
An analytical solution was developed for uniform flux of water and gas into the fracture.
The solution for pressures and saturations in the fracture agrees well with reservoir
simulation. Gravity segregation occurs in moderate-to-high conductivity fractures. Gravity segregation impacts effective fracture conductivity when gas and liquid are
being produced at all water-gas ratios modeled above 2 Bbls per MMscf. More realistic,
non-uniform-flux models were also run with the hydraulic fracture connected to a gas
reservoir producing water. For constant-gas-rate production, differences in pressure
drop between segregated cases and mixed flow cases range up to a factor of two. As the
pressure gradient in the fracture increases above 1 to 2 psi/ft, the amount of segregation
decreases. Segregation is also less for fracture half-length-to-height ratios less than or
close to two. When there is less segregation, the difference in effective conductivity
between the segregated and mixed flow cases is reduced. I also modeled the water
injection and cleanup phases for a typical slickwater fracture treatment both with and
without gravity effects and found that for cases with segregation, effective fracture
conductivity is significantly higher than the conductivity when mixed flow occurs.
Gravity segregation is commonly ignored in design and analysis of hydraulically
fractured gas wells. This work shows that segregation is an important physical process
and it affects effective fracture conductivity significantly. Hydraulic fracture treatments
can be designed more effectively if effective fracture conductivity is known more
accurately.